The present invention relates to a power supply for light sources, particularly for the quick ignition of fluorescent lamps and the like.
A fluorescent lamp is constituted by a glass tube containing a small amount of mercury and argon, internally coated with phosphors, and provided, at its ends, with two electrodes, between which the potential difference required to trigger and maintain the discharge is established.
Each electrode is constituted by a tungsten filament.
The electrodes must usually be preheated to 1200 K. The ignition device must preheat the electrodes and produce the over-voltage required to trigger the discharge.
The ignition device comprises a starter and a ballast. When the mains voltage is applied, the discharge in the neon gas contained in the starter is triggered. The discharge in the starter then ends and the current flows into the filament of the fluorescent lamp. As a consequence of the opening of the contact inside the starter, and due to the considerable self-induction of the ballast, an over-voltage is produced which is capable of triggering the discharge in the fluorescent lamp, producing the arc. The steady-state voltage during the operation of the lamp is lower than the mains voltage and is not sufficient to maintain the discharge in the starter. Accordingly, the starter, after igniting the lamp, remains automatically open.
A drawback of this solution is the need to use a ballast and a starter, with consequent problems related to consumption, cost, and bulk.
The preheating of the electrodes furthermore requires a waiting time, and breakage of the internal filament of the lamp thus prevents its operation.
Finally, the inductive coefficient of the ballast causes a phase shift in the alternating current; if it is not adequately corrected, this shift overloads the circuits with current values that are sometimes twice those necessary. In this case it is therefore necessary to use a rephasing capacitor of adequate value that returns the current to the set values.
From EP-A-0650312 is known a device which makes use of an oscillator for the supply of the fluorescent tube after its ignition. This device makes also use of several rectifier-multiplier stages to increase the voltage for obtaining a high voltage to ignite the tube.
The presence of an oscillator makes the circuitry a complex one, the more so that an amplifier is also provided upstream of the voltage multiplier. The known circuitry provides in a specific complex way for keeping alight the tube and for igniting it.
From FR-A-2645393 is known another device for the ignition of a fluorescent tube, which uses a special circuitry for the polarity inversion of the supply voltage, to avoid the polarizing effect that would render the tube unusable after few hours. Again the circuitry is a complex one because of the need of a device for polarity inversion.